The present invention relates to antenna systems for communicating with satellite based transponders, and more particularly to a beam alignment system method for correcting beam misalignment between a transmit antenna and a receive antenna, where both antennas are carried on a mobile platform and aimed at a target satellite transponder.
Since the advent of radar during World War II, a variety of methods have been developed for tracking moving targets or tracking stationary targets from a moving platform using high gain antennas. In both cases, the pointing angles to the target change with time as the target and/or platform move. With present day satellite communication systems, the use of separate transmit and receive antennas in connection with a mobile terminal used on a mobile platform require very precise alignment between the receive and transmit beams to maintain the beams aimed as closely as possible at a target satellite. Difficulty in maintaining precise alignment between the receive and transmit beams is caused in part because of the physical separation between the antenna apertures. When the transmit antenna beam pointing direction is slaved to that of the receive antenna beam to provide two way communication with the satellite, the physical separation between the two antennas can result in beam misalignment of a sufficient magnitude to cause a degradation in system performance. Another major concern is a regulatory based concern that the beam of the transmit antenna does not interfere with other satellites adjacent the target satellite orbiting along a geosynchronous or non-geosynchronous arc.
With present day systems, the receive antenna is used to track the target satellite using a variety of well known methods such as dithering, conical scanning, monopulse, etc., and the transmit antenna is simply pointed in the direction of the target satellite as determined by the receive antenna tracking system. This is known as the xe2x80x9cslavedxe2x80x9d approach. A problem, however, is that there is presently no method to detect and correct for transmit/receive antenna misalignment for remote mobile terminals without removing them from service. Of course, if the misalignment becomes too large, then it is simple to detect because the transmit antenna will no longer point at the target satellite and the communication uplink to the satellite will no longer function.
With transmit and receive antennas located on a mobile platform, an alternative method of pointing the transmit antenna is simply using an xe2x80x9copen loopxe2x80x9d pointing approach, wherein the position and attitude of the mobile platform (gleaned from an on-board navigation system of the mobile platform) and a known position of the target satellite are used to determine the direction of pointing of the transmit antenna. The shortcoming of the open loop pointing approach is that its pointing accuracy is limited by the accuracy of the on-board navigation system. The information provided by the on-board navigation system is typically not quite accurate enough to achieve the needed pointing accuracy of the transmit antenna, which is on the order of about 0.1xc2x0. It would be very difficult or even impossible to maintain this degree of beam alignment between the transmit and receive antennas using the slaved beam approach, over the life of the system, without requiring periodic realignment of the transmit antenna. In aviation applications, the transmit and receive antennas of a satellite communications system will need to be mounted on the fuselage of an aircraft and separated typically by several feet to provide sufficient RF isolation. Antennas installed on aircraft operated by commercial airlines are expected to function for years of trouble free service with little or no costly maintenance. Under these conditions, it would be difficult or impossible to guarantee mechanical and electrical transmit/receive antenna alignment to within 0.1xc2x0.
Therefore, some form of beam alignment system is needed to correct for misalignment of a transmit antenna beam relative to a receive antenna beam when the receive and transmit antennas are used to communicate with an orbiting satellite. Such a system and method is further required which can be implemented without requiring significant additional costly and complex equipment to be carried on a mobile platform, and which can be expected to function with little or no maintenance.
The present invention is directed to a system and method for correcting for misalignment between a transmit antenna beam and a receive antenna beam of transmit and receive antennas operating on a host platform and communicating with a satellite communications system. It is anticipated that the present invention will find particular utility in connection with transmit and receive antennas carried on mobile platforms such as aircraft, however, the invention is equally applicable to a stationary host platform or to other forms of mobile platforms such as ships or moving land based vehicles.
The present invention, in one preferred embodiment, determines a beam center of the receive antenna tracking a target satellite through one of a plurality of well known methods. Once the beam center of the receive antenna is determined, the transmit antenna is pointed in accordance with pointing angles determined for the receive antenna, as described above. One of a plurality of well known methods is used to then determine a beam center of the transmit antenna as the transmit antenna transmits information to the target satellite. Sequential lobing is one preferred method which is used to determine the beam center of the transmit antenna.
A parameter of the signal quality or signal strength of the received signal at a ground station in communication with the target satellite is then measured and recorded for each one of a plurality of points in the transmit antenna sequential lobing pattern. The recorded signal strength or quality measurements are then transmitted to the host platform via the target satellite. From this information, the alignment error between the transmit and receive beams can be determined. Alternatively, this information may be used to determine the alignment error between the transmit and receive antennas on the ground. The correction can then be transmitted via the target satellite to the host platform.
It will be appreciated that the system and method of the present invention may be used with virtually any well known method for determining the antenna pointing angles to the target satellite. Sequential lobing is one preferred method. Other well known methods include beam dithering, conical scanning and monopulse tracking. The parameter measured by the receiver of the ground station and fed back to the host platform for finding the beam center of the transmit beam comprises signal strength in one preferred embodiment of the invention, but other possible parameters are signal-to-noise ratio (S/N), carrier-to-noise ratio (C/N) or energy per bit divided by noise spectral density (Eb/No).